Vari-focal lens system

ABSTRACT

A compact vari-focal lens system comprising, in the order from the object side, a first lens unit having a positive refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power, and uses at least one radial type graded refractive index lens element arranged at the image side location in the second lens unit. The vari-focal lens system has a high vari-focal ratio and aberrations corrected favorably over the entire vari-focal range from the wide position to the tele position, and consists of a very small number of lens elements.

This is a continuation of application Ser. No. 07/736,999, filed Jul. 8,1991 now U.S. Pat. No. 5,196,962

BACKGROUND OF THE INVENTION

a) Field of the invention:

The present invention relates to a compact vari-focal lens system forlens shutter cameras which comprises a small number of lens elements andhas a high vari-focal ratio.

b) Description of the prior art:

In the recent years, the prevalence of compact cameras has produceddemands for vari-focal lens systems which are compact, light in weightand have high-grade specifications. Especially for use with the lensshutter cameras which do not permit exchange of lens systems, it isdemanded to develop compact vari-focal lens systems having vari-focalratios of 2 and higher.

As a vari-focal lens system which is designed so as to satisfy thedemand for compact and light-weight design, there is already known alens system which consists of a front lens unit having a positiverefractive power and a rear lens unit having a negative refractivepower, and is adapted so as to perform variation of focal length byvarying the airspace reserved between these two lens units. However, itis difficult to design this vari-focal lens system so as to have avari-focal ratio of 2 or higher and high-grade specifications since thefocal length of this lens system is varied by moving the rear lens unitwhich has a magnification higher than 1× and the negative refractivepower.

As vari-focal lens systems which have solved this problem or aredesigned so as to have high vari-focal ratios, there are known the lenssystems disclosed by Japanese Patent Kokai Publication No. Sho 63-43115and Japanese Patent Kokai Publication No. Hei 1-252916. Each of thesevari-focal lens systems consists, in the order from the object side, ofthree positive, positive and negative lens units or four positive,negative, positive and negative lens units, and has a vari-focal ratioon the order of 3. However, this vari-focal lens system comprises atleast eleven lens elements even when aspherical surfaces are usedtherein and is not sufficiently satisfactory from the viewpoint ofcompact and lightweight design.

On the other hand, attentions are now paid to the lens systems whichadopt graded refractive index lens elements. Graded refractive indexlens elements are classified, dependently on directions of refractiveindex distributions, into the axial type which has a refractive indexdistribution in the direction along the optical axis, the radial typewhich has a refractive index distribution in the direction perpendicularto the optical axis, and so on. Especially the radial type gradedrefractive index lens element is expected to exhibit an effect to makelens systems compacter and lighter in weight.

The lens systems disclosed by Japanese Patent Kokai Publication No. Sho63-159818 and Japanese Patent Kokai Publication No. Sho 63-161423 areknown as the vari-focal lens systems for lens shutter cameras havingvari-focal ratios of approximately 3 and using graded refractive indexlens elements. Each of these lens systems is designed so as to suppressvariations of aberrations during variation of focal lengths by using theaxial type graded refractive index lens element, but comprises elevenlens elements and is not sufficiently satisfactory from the viewpoint ofthe compact and light-weight design.

Further, the lens systems disclosed by Japanese Patent Kokai PublicationNo. Sho 61-126515, Japanese Patent Kokai Publication No. Sho 61-148414and so on are vari-focal lens systems for lens shutter cameras using theradial type graded refractive index lens elements and comprising lenselements in small numbers, the minimum of which is only five. However,these lens systems have low vari-focal ratios of 1.5 to 2 and are stillunsatisfactory from the viewpoint of the high-grade specifications.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a vari-focallens system which has a vari-focal ratio as high as 3, aberrationscorrected sufficiently favorably over the entire vari-focal range fromthe wide position to the tele position, a short total length and acomposition consisting of a very small number of lens elements.

The vari-focal lens system according to the present invention comprises,in the order from the object side, a first lens unit having a positiverefractive power, a second lens unit having a positive refractive powerand a third lens unit having a negative refractive power, and uses insaid second lens unit at least one graded refractive index lens elementwhich has a refractive index distribution in the direction perpendicularto the optical axis (radial type GRIN lens element).

For the vari-focal lens system according to the present invention, it isdesirable to design the second lens unit so as to comprise, in the orderfrom the object side, at least a lens element having a negativerefractive power and a lens element having a positive refractive power,and have a positive refractive power as a whole.

Since rays can be refracted in the diverging directions by arranging thelens element having the negative refractive power at the object sidelocation in the second lens unit as described above and the focal lengthof the vari-focal lens system, especially at the tele position thereof,can be prolonged by designing the lens system so as to perform variationof focal length by moving the individual lens units independently, it ispossible to obtain a lens system having a high vari-focal ratio.Further, it is effective for shortening the total length of thevari-focal lens system to select the telephoto type power distribution.For this reason, the lens unit having the negative refractive power isarranged at the image side location in the vari-focal lens systemaccording to the present invention.

By composing the vari-focal lens system according to the presentinvention as described above, it is possible to enhance the vari-focalratio of the lens system and shorten the total length thereof to acertain degree. However, the vari-focal lens system currently having thesimplest composition and consisting only of homogenous lens elementscomprises eleven lens elements even when aspherical surfaces are usedtherein and it is very difficult to reduce the number of the lenselements to 2/3 thereof or so.

The present invention has succeeded in remarkably reducing the number ofthe lens elements by using, in the vari-focal lens system, at least oneradial GRIN lens element which has a refractive index distribution inthe direction perpendicular to the optical axis.

The radial type GRIN lens element used in the vari-focal lens systemaccording to the present invention has a refractive index distributionfor the ray having the standard wavelength which is expressed by theformula shown below:

    n(r)=N.sub.0 +N.sub.1 r.sup.2 +N.sub.2 r.sup.4 +N.sub.3 r.sup.6 + . . .

the reference symbol N₀ represents the refractive index for the rayhaving the standard wavelength of the radial GRIN lens element asmeasured on the optical axis, the reference symbol r designates thedistance as measured from the optical axis to a portion of interest ofthe radial GRIN lens element in the radial direction, the referencesymbol n(r) denotes the refractive index for the ray having the standardwavelength as measured at the portion located at the distance r, and thereference symbols N₁, N₂, N₃, . . . represent the refractive indexdistribution coefficients of the second, fourth, sixth, . . . ordersrespectively for the ray having the standard wavelength.

Since the GRIN lens element has a refractive power also of the mediumthereof, it is possible, by imparting a refractive power of surface anda refractive power of medium which have the same sign, to equalize thetotal refractive power of the GRIN lens element to that of a homogenouslens element even when the GRIN lens element has a curvature on thesurface thereof which is lower than that on the surface of thehomogenous lens element.

Further, the refractive power of the medium of the GRIN lens element isdependent mainly on the coefficient of the second order N₁ and thethickness of the GRIN lens element as measured on the optical axis.Since the refractive power of the medium acts as a negative power whenthe coefficient N₁ is positive or acts as positive power when thecoefficient N₁ is negative, it is possible to control the refractivepower of the GRIN lens element by both the surface and the mediumthereof so as to enhance the freedom for correction of aberrations andreduce the number of the lens elements required for composing the lenssystem.

Furthermore, since the GRIN lens element has the refractive power of themedium thereof, it is usable for correcting Petzval's sum which cannotbe corrected with aspherical surfaces.

The Petzval's sum of the GRIN lens element, considered independently ofthe lens system, is expressed by the following formula:

    φ.sub.S /N.sub.0 +φ.sub.M /N.sub.0.sup.2

wherein the reference symbol φ_(S) represents the refractive power ofthe surface of the GRIN lens element and the reference symbol φ_(M)designates the refractive power of the medium of the GRIN lens element.

As is clear from the formula mentioned above, the GRIN lens elementpermits adjusting Petzval's sume with a certain degree of freedom bycontrolling the refractive power of the surface and the refractive powerof the medium thereof. By using the GRIN lens element in the vari-focallens system according to the present invention, it is therefore possibleto favorably correct the Petzval's sum of the lens system even when itis composed of a reduced number of lens elements.

Moreover, when the GRIN lens element has refractive index distributionsof both the medium and the surface thereof, it permits correctingaberrations with the correction terms of the surface which havebehaviors different from those of a homogenous lens element, therebymaking it possible to further improve optical performance of thevari-focal lens system.

Since the refractive power of the GRIN lens element can be shared withthe medium thereof as described above, it is effective to use the GRINlens element in a lens unit which has an especially strong refractivepower. For this reason, the present invention uses at least one GRINlens element in the second positive lens unit which has the strongestrefractive power in the vari-focal lens system.

The present invention has succeeded in obtaining a vari-focal lenssystem which has a high vari-focal ratio and has a short total length byselecting the composition described above, and remarkably reducing thenumber of lens elements while maintaining the favorable opticalperformance of the lens system by using the GRIN lens element in thesecond lens unit having the strongest refractive power.

For the vari-focal lens system according to the present inventiondescribed above, it is desirable to design the GRIN lens element to bearranged in the second lens unit so as to satisfy the followingcondition (1):

    N.sub.1(2) >0                                              (1)

wherein the reference symbol N₁(2) represents the refractive indexdistribution coefficient of the second order of the GRIN lens element.

When two or more GRIN lens elements are to be used in the second lensunit, it is sufficient that at least one of the GRIN lens elementsatisfies the above-mentioned condition (1). The condition (1) definesthe refractive power of the medium of the GRIN lens element and meansthat the medium is to have a positive refractive power. When the GRINlens element is designed so as to satisfy the condition (1), thepositive refractive power required for the second lens unit can beshared with the medium of the GRIN lens element and Petzval's sum can bereduced, whereby the use of the GRIN lens element makes it possible toreduce the number of lens elements required to compose the vari-focallens system.

If the upper limit of the condition (1) is exceeded, the medium of theGRIN lens element will have a negative refractive power. Accordingly, itwill be necessary to strengthen the refractive powers of the homogenouslens elements for obtaining the positive refractive power required forthe second lens unit, thereby making it impossible to reduce the numberof the lens elements required to compose the vari-focal lens system.

The GRIN lens element which is designed so as to satisfy the condition(1) may be arranged at any location in the second lens unit, but shoulddesirably be arranged at the image side location for allowing the effectof the GRIN lens element to be exhibited sufficiently. In order toenhance the vari-focal ratio of the lens system according to the presentinvention, rays must be refracted by the second lens unit in thediverging directions so that the rays are high on the lens elementsarranged on the image side of the second lens unit. In addition, themarginal ray has a large angle of refraction on the lens elementsarranged on the image side of the second lens unit at the wide position.It is therefore most effective for correction of spherical aberration toarrange the GRIN lens element at the image side location in the secondlens unit as described above.

When the GRIN lens element is arranged at the image side location in thesecond lens unit as described above, it is desirable that the GRIN lenselement satisfies the following condition (2):

    K·N.sub.2(1) >0                                   (2)

wherein the reference symbol N₂ (1) represents the refractive indexdistribution coefficient of the fourth order of the GRIN lens elementarranged at the image side location in the second lens unit, and thereference symbol k designates a constant having a value of +1 when theeither of the surfaces of the GRIN lens element whichever has the highercurvature is convex or a value of -1 when either of the surfaces of theGRIN lens element whichever has the higher curvature is concave.

The condition (2) is required for imparting the GRIN lens element arefractive index distribution which is effective for correctingespecially spherical aberration when the GRIN lens element is arrangedat the most image side location in the second lens unit.

The GRIN lens element can correct aberrations with the surface, themedium and the correction term of the refractive index distributionimparted to the surface having a behavior different from that of ahomogenous lens element. The condition (2) is required for determiningthe sign of the spherical aberration to be produced by the medium of theGRIN lens element and is adopted for favorably correcting aberrations inthe vari-focal lens system as a whole.

The vari-focal lens system according to the present invention as a wholeproduces spherical aberration on the negative side over the entirevari-focal range from the wide position to the tele position. When theGRIN lens element is to be arranged at the image side location in thesecond lens unit for correcting the spherical aberration, the GRIN lenselement is designed so as to satisfy the condition (1) or have apositive refractive power. When the constant k has a value of +1, i.e.,when either of the surfaces of the GRIN lens element whichever has thehigher curvature is convex, negative spherical aberration is produced bythe surface of the GRIN lens element and positive spherical aberrationis produced by the correction terms of the refractive index distributionimparted to the surface having a behavior different from that of ahomogenous lens element. However, since the number of the lens elementsis reduced and the positive refractive power of the second lens elementis shared with the medium of the GRIN lens element in the vari-focallens system, the positive spherical aberration produced by theabove-mentioned correction terms is predominant, thereby causingovercorrection of spherical aberration in the vari-focal lens system asa whole. Therefore, it is desirable that the GRIN lens element satisfiesthe condition (2) so as to produce negative spherical aberration by themedium thereof, whereby the spherical aberration is corrected favorablyin the vari-focal lens system as a whole.

Further, when the constant k has a value of -1, i.e., when either of thesurface of the GRIN lens element whichever has the higher curvature isconcave, positive spherical aberration is produced by the surface of theGRIN lens element and negative spherical aberration is produced by thecorrection terms of said surface. Therefore, the spherical aberrationcan be corrected favorably in the vari-focal lens system as a whole whenthe GRIN lens element is designed so as to satisfy the condition (2) orproduce positive spherical aberration by the medium thereof.

If the upper limit of the condition (2) is exceeded, the sphericalaberration will be unbalanced or cannot be corrected favorably in thevari-focal lens system as a whole.

In addition, in the second lens unit of the vari-focal lens systemaccording to the present invention, the object side surface of thenegative lens element arranged at the object side location is concavetoward the object side to enhance the vari-focal ratio of the vari-focallens system and correct the spherical aberration in the lens system as awhole which is apt to be produced on the negative side.

In the vari-focal lens system according to the present invention whichis composed of the three lens units as described above, the second lensunit having the positive refractive power may be divided into an objectside lens unit having a negative refractive power and an image side lensunit having a positive lens unit so that the vari-focal lens system iscomposed of four lens units. Speaking more concretely, the vari-focallens system according to the present invention may comprise a first lensunit having a positive refractive power, a second lens unit having anegative refractive power, a third lens unit having a positiverefractive power and a fourth lens unit having a negative refractivepower.

For the vari-focal lens system which comprises the four lens units asdescribed above, it is sufficient to use at least one radial GRIN lenselement in the third lens unit having the positive refractive power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 6 show sectional views illustrating compositions ofEmbodiments 1 through 6 respectively of the vari-focal lens systemaccording to the present invention;

FIG. 7, FIG. 8 and FIG. 9 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 1 of the present invention;

FIG. 10, FIG. 11 and FIG. 12 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 2 of the present invention:

FIG. 13, FIG. 14 and FIG. 15 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 3 of the pre sent invention;

FIG. 16, FIG. 17 and FIG. 18 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 4 of the present invention;

FIG. 19, FIG. 20 and FIG. 21 show graphs visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 5 of the present invention; and

FIG. 22, FIG. 23 and FIG. 24 show curves visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of the Embodiment 6 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the vari-focal lens system according to the present invention willbe described more detailedly below with reference to the preferredembodiments illustrated in the accompanying drawings and given in theform of the following numerical data:

    ______________________________________                                        Embodiment 1                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω = 62.8°˜23.8°                                    r.sub.1 =                                                                           -82.8124                                                                      d.sub.1 =1.0000                                                                              n.sub.1 = 1.83400                                                                          ν.sub.1 = 37.16                          r.sub.2 =                                                                           142.5629                                                                      d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           28.1579                                                                       d.sub.3 = 3.1200                                                                             n.sub.2 = 1.65160                                                                          ν.sub.2 = 58.52                          r.sub.4 =                                                                           -71.3782                                                                      d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -20.6631                                                                      d.sub.5 = 1.7665                                                                             n.sub.3 = 1.77250                                                                          ν.sub.3 = 49.66                          r.sub.6 =                                                                           -185.0591                                                                     d.sub.6 = 0.5056                                                        r.sub.7 =                                                                           123.4202                                                                      d.sub.7 = 5.1610                                                                             n.sub.4 = 1.84666                                                                          ν.sub.4 = 23.78                          r.sub.8 =                                                                           -254.9242                                                                     d.sub.8 = 0.5079                                                        r.sub.9 =                                                                           ∞ (stop)                                                                d.sub.9 = 2.9876                                                        r.sub.10 =                                                                          79.1022                                                                       d.sub.10 = 6.0190                                                                            n.sub.5 (GRIN lens)                                      r.sub.11 =                                                                          -24.4531                                                                      d.sub.11 = D.sub.2 (variable)                                           r.sub.12 =                                                                          -89.5236                                                                      d.sub.12 = 2.3072                                                                            n.sub.6 = 1.80518                                                                          ν.sub.6 = 25.43                          r.sub.13 =                                                                          -37.1796                                                                      d.sub.13 = 0.1500                                                       r.sub.14 =                                                                          -58.1985                                                                      d.sub.14 = 1.0017                                                                            n.sub.7 = 1.69680                                                                          ν.sub.7 = 56.49                          r.sub.15 =                                                                          64.5623                                                                       d.sub.15 = 4.5021                                                       r.sub.16 =                                                                          -23.3155                                                                      d.sub.16 = 1.0011                                                                            n.sub.8 = 1.72916                                                                          ν.sub.8 = 54.68                          r.sub.17 =                                                                          -92.8175                                                                ______________________________________                                        f       36.2          60.5    101.1                                           D.sub.1 1.501         6.090   9.989                                           D.sub.2 16.147        7.174   1.104                                           GRIN lens                                                                               N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.70698 × 10.sup.-3                                                                 -0.36536 × 10.sup.-6                    C line    1.60425, -0.71533 × 10.sup.-3                                                                 -0.44967 × 10.sup.-6                    F line    1.61448, -0.68750 × 10.sup.-3                                                                 -0.16863 × 10.sup.-6                    N.sub.1 (2) = -0.70698 × 10.sup.-3                                      K · N.sub.2 (1) = -0.36536 × 10.sup.-6                         ______________________________________                                        Embodiment 2                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω = 61.8°˜23.8°                                    r.sub.1 =                                                                           211.6243                                                                      d.sub.1 = 1.0000                                                                           n.sub.1 = 1.83400                                                                          ν.sub.1 = 37.16                            r.sub.2 =                                                                           46.8544                                                                       d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           22.0690                                                                       d.sub.3 = 3.2057                                                                           n.sub.2 = 1.61700                                                                          ν.sub.2 = 62.79                            r.sub.4 =                                                                           -116.7022                                                                     d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -17.8856                                                                      d.sub. 5 = 7.6485                                                                          n.sub.3 (GRIN lens 1)                                      r.sub.6 =                                                                           -388.0124                                                                     d.sub.6 = 0.8632                                                        r.sub.7 =                                                                           ∞ (stop)                                                                d.sub.7 = 0.8967                                                        r.sub.8 =                                                                           57.8611                                                                       d.sub.8 = 3.7235                                                                           n.sub.4 (GRIN lens 2)                                      r.sub.9 =                                                                           -17.2617                                                                      d.sub.9 = D.sub.2 (variable)                                            r.sub.10 =                                                                          -88.8573                                                                      d.sub.10 = 3.0258                                                                          n.sub.5 = 1.76182                                                                          ν.sub.5 = 26.55                            r.sub.11 =                                                                          -37.2598                                                                      d.sub.11 = 0.1572                                                       r.sub.12 =                                                                          -63.3184                                                                      d.sub.12 = 1.0017                                                                          n.sub.6 = 1.69680                                                                          ν.sub.6 = 56.49                            r.sub.13 =                                                                          88.8136                                                                       d.sub.13 = 4.8000                                                       r.sub.14 =                                                                          -21.9171                                                                      d.sub.14 = 1.0011                                                                          n.sub.7 = 1.72916                                                                          ν.sub.7 = 54.68                            r.sub.15 =                                                                          -104.0855                                                               ______________________________________                                        f       36.2          60.5     101.2                                          D.sub.1 1.550         5.369   9.355                                           D.sub.2 17.058        7.688   1.200                                           GRIN lens 1                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.77250, -0.15036 × 10.sup.-3                                                                 0.17762 × 10.sup.-5                     C line    1.76780, -0.14426 × 10.sup.-3                                                                 0.16430 × 10.sup.-5                     F line    1.78336, -0.16458 × 10.sup.-3                                                                 0.20870 × 10.sup.-5                     GRIN lens 2                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.57582 × 10.sup.-3                                                                 -0.31558 × 10.sup.-6                    C line    1.60418, -0.59022 × 10.sup.-3                                                                 -0.34343 × 10.sup.-6                    F line    1.61441, -0.54223 × 10.sup.-3                                                                 -0.25061 × 10.sup.-6                    N.sub.1 (2) = -0.15036 × 10.sup.-3 (GRIN lens 1)                        N.sub.1 (2) = -0.57582 × 10.sup.-3 (GRIN lens 2)                        K · N.sub.2 (1) -0.31558 × 10.sup.-6                           ______________________________________                                        Embodiment 3                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω = 61.8°˜24.2°                                    r.sub.1 =                                                                           479.0916                                                                      d.sub.1 = 0.9060                                                                             n.sub.1 = 1.85026                                                                          ν.sub.1 = 32.28                          r.sub.2 =                                                                           62.8896                                                                       d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           24.5476                                                                       d.sub.3 = 3.0039                                                                             n.sub.2 = 1.61700                                                                          ν.sub.2 = 62.79                          r.sub.4 =                                                                           -142.6960                                                                     d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -20.2579                                                                      d.sub.5 = 8.6330                                                                             n.sub.3 (GRIN lens 1)                                    r.sub.6 =                                                                           -123.3366                                                                     d.sub.6 = 0.5113                                                        r.sub.7 =                                                                           ∞ (stop)                                                                d.sub.7 = 1.6204                                                        r.sub.8 =                                                                           65.5250                                                                       d.sub.8 = 5.3893                                                                             n.sub.4 (GRIN lens 2)                                    r.sub.9 =                                                                           -25.6373                                                                      d.sub.9 = D.sub.2 (variable)                                            r.sub.10 =                                                                          -24.6203                                                                      d.sub.10 = 3.4433                                                                            n.sub.5 = 1.80518                                                                          ν.sub.5 = 25.43                          r.sub.11 =                                                                          -18.4364                                                                      d.sub.11 = 2.3876                                                       r.sub.12 =                                                                          -18.3338                                                                      d.sub.12 = 1.7003                                                                            n.sub.6 = 1.77250                                                                          ν.sub.6 = 49.66                          r.sub.13 =                                                                          -7442.4481                                                              ______________________________________                                        f       36.2          60.5    101.1                                           D.sub.1 1.349         6.162   11.259                                          D.sub.2 19.754        8.789   1.125                                           GRIN lens 1                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.77250, -0.11630 × 10.sup.-3                                                                 -0.23605 × 10.sup.-6                    C line    1.76780, -0.10633 × 10.sup.-3                                                                 -0.30687 × 10.sup.-6                    F line    1.78336, -0.13955 × 10.sup.-3                                                                 -0.70815 × 10.sup.-7                    GRIN lens 2                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.93189 × 10.sup.-3                                                                 -0.02954 × 10.sup.-5                    C line    1.60418, -0.94521 × 10.sup.-3                                                                 - 0.21478 × 10.sup.-5                   F line    1.61441, -0.90083 × 10.sup.-3                                                                 -0.19732 × 10.sup.-5                    N.sub.1 (2) = -0.11630 × 10.sup.-3 (GRIN lens 1)                        N.sub.1 (2) = -0.93189 × 10.sup.-3 (GRIN lens 2)                        K · N.sub.2 (1) = -0.20954 × 10.sup.-5                         ______________________________________                                        Embodiment 4                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω = 61.8°˜24.2°                                    r.sub.1 =                                                                           -1911.4184                                                                    d.sub.1 = 1.0000                                                                             n.sub.1 = 1.85026                                                                          ν.sub.1 = 32.28                          r.sub.2 =                                                                           60.9501                                                                       d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           22.7908                                                                       d.sub.3 = 2.9203                                                                             n.sub.2 = 1.69680                                                                          ν.sub.2 = 56.49                          r.sub.4 =                                                                           -213.5384                                                                     d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -19.1242                                                                      d.sub.5 = 2.4422                                                                             n.sub.3 (GRIN lens 1)                                    r.sub.6 =                                                                           -96.2459                                                                      d.sub.6 = 0.7132                                                        r.sub.7 =                                                                           ∞ (stop)                                                                d.sub.7 = 1.9319                                                        r.sub.8 =                                                                           188.2584                                                                      d.sub.8 = 6.4054                                                                             n.sub.4 (GRIN lens 2)                                    r.sub.9 =                                                                           -18.7161                                                                      d.sub.9 = D.sub.2 (variable)                                            r.sub.10 =                                                                          -18.0399                                                                      d.sub.10 = 2.5828                                                                            n.sub.5 = 1.84666                                                                          ν.sub.5 = 23.78                          r.sub.11 =                                                                          -16.1038                                                                      d.sub.11 = 1.7151                                                       r.sub.12 =                                                                          -18.6077                                                                      d.sub.12 = 1.4123                                                                            n.sub.6 = 1.69680                                                                          ν.sub.6 = 56.49                          r.sub.13 =                                                                          -1033.1754                                                              ______________________________________                                        f       36.2          60.5    101.1                                           D.sub.1 1.349         6.469   11.685                                          D.sub.2 20.218        9.083   1.125                                           GRIN lens 1                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.78590, 0.20685 × 10.sup.-3                                                                  0.12458 × 10.sup.-5                     C line    1.78059, 0.23788 × 10.sup.-3                                                                  0.97884 × 10.sup.-6                     F line    1.79837, 0.13445 × 10.sup.-3                                                                  0.18687 × 10.sup.-5                     GRIN lens 2                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.88234 × 10.sup.-3                                                                 -0.14584 × 10.sup.-5                    C line    1.60418, -0.89125 × 10.sup.-3                                                                 -0.15556 × 10.sup.-5                    F line    1.61441, -0.86154 × 10.sup.-3                                                                 -0.12315 × 10.sup.-5                    N.sub.1 (2) = -0.88234 × 10.sup.-3                                      K · N.sub.2 (1) = -0.14584 × 10.sup.-5                         ______________________________________                                        Embodiment 5                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω = 61.8°˜24.2°                                    r.sub.1 =                                                                           943.2571                                                                      d.sub.1 = 0.9060                                                                             n.sub.1 = 1.84666                                                                          ν.sub.1 = 23.78                          r.sub.2 =                                                                           74.9401                                                                       d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           19.6655                                                                       d.sub.3 = 3.0039                                                                             n.sub.2 = 1.69680                                                                          ν.sub.2 = 56.49                          r.sub.4 =                                                                           106.6352                                                                      d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -33.6973                                                                      d.sub.5 = 4.9412                                                                             n.sub.3 (GRIN lens 1)                                    r.sub.6 =                                                                           -541.0911                                                                     d.sub.6 = 0.5113                                                        r.sub.7 =                                                                           ∞ (stop)                                                                d.sub.7 = 1.9941                                                        r.sub.8 =                                                                           159.8040                                                                      d.sub.8 = 6.8298                                                                             n.sub.4 (GRIN lens 2)                                    r.sub.9 =                                                                           -19.9268                                                                      d.sub.9 = D.sub.2 (variable)                                            r.sub.10 =                                                                          -18.1090       (aspherical surface)                                           d.sub.10 = 3.7460                                                                            n.sub.5 = 1.84666                                                                          ν.sub.5 = 23.78                          r.sub.11 =                                                                          -17.3972                                                                      d.sub.11 = 2.2430                                                       r.sub.12 =                                                                          -15.2734                                                                      d.sub.12 = 1.5387                                                                            n.sub.6 = 1.69680                                                                          ν.sub.6 = 56.49                          r.sub.13 =                                                                          -148.0065                                                               ______________________________________                                        f       36.2          60.5    101.1                                           D.sub.1 1.349         7.157   11.821                                          D.sub.2 15.320        7.051   1.125                                           aspherical surface coefficient                                                P = 1,  A.sub.4 = 0.20309 × 10.sup.-4                                   A.sub.6 = 0.12986 × 10.sup.-6, A.sub.8 = -0.11550                       × 10.sup.-8                                                             A.sub.10 = 0.69082 × 10.sup.-11                                         GRIN lens 1                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.78590, 0.20939 × 10.sup.-3                                                                  0.55808 × 10.sup.-5                     C line    1.78059, 0.22665 × 10.sup.-3                                                                  0.54936 × 10.sup.-5                     F line    1.79837, 0.16912 × 10.sup.-3                                                                  0.57843 × 10.sup.-5                     GRIN lens 2                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.75272 × 10.sup.-3                                                                 -0.23447 × 10.sup.-5                    C line    1.60418, -0.76096 × 10.sup.-3                                                                 -0.24361 × 10.sup.- 5                   F line    1.61441, -0.73349 × 10.sup.-3                                                                 -0.21315 × 10.sup.-5                    N.sub.1 (2) = -0.75272 × 10.sup.-3                                      K · N.sub.2 (1) = -0.23447 × 10.sup.-5                         Embodiment 6                                                                  f = 35˜105 mm, F/4.5˜F/6.5                                        2ω= 61.8°˜24.2°                                     r.sub.1 =                                                                           -4346.0327                                                                    d.sub.1 = 1.0000                                                                             n.sub.1 = 1.85026                                                                          ν.sub.1 = 32.28                          r.sub.2 =                                                                           62.3955                                                                       d.sub.2 = 0.1200                                                        r.sub.3 =                                                                           20.5429                                                                       d.sub.3 = 2.9203                                                                             n.sub.2 = 1.69680                                                                          ν.sub.2 = 56.49                          r.sub.4 =                                                                           -1500.3464                                                                    d.sub.4 = D.sub.1 (variable)                                            r.sub.5 =                                                                           -18.6204                                                                      d.sub.5 = 1.6757                                                                             n.sub.3 (GRIN lens 1)                                    r.sub.6 =                                                                           -51.5666                                                                      d.sub.6 = D.sub.2 (variable)                                            r.sub.7 =                                                                           ∞  (stop)                                                               d.sub.7 = 1.8565                                                        r.sub.8 =                                                                           11621.8599                                                                    d.sub.8 = 6.1924                                                                             n.sub.4 (GRIN lens 2)                                    r.sub.9 =                                                                           -19.4997                                                                      d.sub.9 = D.sub.3 (variable)                                            r.sub.10 =                                                                          -17.0949                                                                      d.sub.10 = 2.3098                                                                            n.sub.5 = 1.84666                                                                          ν.sub.5 = 23.78                          r.sub.11 =                                                                          -15.9798                                                                      d.sub.11 = 1.3292                                                       r.sub.12 =                                                                          -18.8436                                                                      d.sub.12 = 1.4123                                                                            n.sub.6 = 1.69680                                                                          ν.sub.6 = 56.49                          r.sub.13 =                                                                          6132.4348                                                               ______________________________________                                        f       36.2          60.5    101.1                                           D.sub.1 1.349         5.843   11.484                                          D.sub.2 1.000         0.800   0.500                                           D.sub.3 18.835        8.672   1.121                                           GRIN lens 1                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.78590, 0.20349 × 10.sup.-3                                                                   0.23521 × 10.sup.-7                    C line    1.78059, 0.21249 × 10.sup.-3                                                                  -0.77283 × 10.sup.-7                    F line    1.79837, 0.18248 × 10.sup.-3                                                                   0.25873 × 10.sup.-6                    GRIN lens 2                                                                             N.sub.0  N.sub.1      N.sub.2                                       d line    1.60729, -0.90307 × 10.sup.-3                                                                 -0.71089 × 10.sup.-6                    C line    1.60418, -0.90310 × 10.sup.-3                                                                 -0.77362 × 10.sup.-6                    F line    1.61441, -0.90301 × 10.sup.-3                                                                 -0.56453 × 10.sup.-6                    N.sub.1 (2) = -0.90307 × 10.sup.-3                                      K · N.sub.2 (1) = -0.71089 × 10.sup.-6                         ______________________________________                                    

wherein the reference symbols r₁, r₂, . . . represent the radii ofcurvature on the surfaces of the respective lens elements, the referencesymbols d₁, d₂, . . . designate the thicknesses of the respective lenselements and the airspaces reserved therebetween, the reference symbolsn₁, n₂, . . . denote the refractive indices of the respective lenselements, and the reference symbols ν₁, ν₂, . . . represent the Abbe'snumbers of the respective lens elements.

The Embodiment 1 has the composition illustrated in FIG. 1, wherein thevari-focal lens system according to the present invention comprises, inthe order from the object side, a first lens unit I having a positiverefractive power, a second lens unit II having a positive refractivepower and a third lens unit II having a negative refractive power, andthe lens element arranged at the image side location in the second lensunit is designed as the GRIN lens element satisfying the conditions (1)and (2) .

Owing to the adoption of the composition and the GRIN lens elementdescribed above, the Embodiment 1 has a vari-focal ratio ofapproximately 3 and nevertheless consists of eight lens elements, whichnumber is smaller by three than the number of the lens elements requiredfor composing the conventional vari-focal lens system. Further, theEmbodiment 1 is designed as a compact vari-focal lens system having ashort total length, i.e., a telephoto ratio of approximately 1.51 at thewide position thereof.

The Embodiment 2 has the composition illustrated in FIG. 2, wherein thevari-focal lens system according to the present invention comprises, inthe order from the object side, a first lens unit I having a positiverefractive power, a second lens unit II having a positive refractivepower and a third lens unit III having a negative refractive power, andthe lens element arranged at the image side location in the second lensunit is designed as the GRIN lens element satisfying the conditions (1)and (2).

In the Embodiment 2, the negative lens element arranged at the objectside location in the second lens unit is also designed as a GRIN lenselement and the second lens unit is composed only of these two GRIN lenselements, whereby the vari-focal lens system as a whole consists ofseven lens elements and has a telephoto ratio of 1.47 at the wideposition thereof.

The GRIN lens element arranged at the object side location in the secondlens unit of the Embodiment 2 is made of a medium having a positiverefractive power so that the positive refractive power of the secondlens unit II is shared with the medium of the GRIN lens element.Further, a positive value is selected for the refractive powerdistribution coefficient N₂ of the GRIN lens element arranged at theobject side location so that spherical aberration is corrected favorablyin the vari-focal lens system as a whole by producing positive sphericalaberration by the medium of the GRIN lens element.

The Embodiment 3 has the composition shown in FIG. 3, wherein thevari-focal lens system according to the present invention comprises, inthe order from the object side, a first lens unit I having a positiverefractive power, a second lens unit I having a positive refractivepower and a third lens unit II having a negative refractive power, andthe lens element arranged at the image side location in the second lensunit is designed as the GRIN lens element satisfying the conditions (1)and (2).

In the Embodiment 3, as in the case of the Embodiment 2, the negativelens element arranged at the object side location in the second lensunit II is also designed as a GRIN lens element and the second lens unitII is composed only of the two GRIN lens elements. Further, the thirdlens unit is also composed of two lens elements, whereby the vari-focallens system preferred as the Embodiment 3 as a whole consists of sixlens elements. The Embodiment 3 consists of lens elements in a numberequal to half the number of the lens elements required for composing theconventional vari-focal lens system and has a telephoto ratio of 1.54 atthe wide position thereof.

In the Embodiment 3, the GRIN lens element arranged at the object sidelocation in the second lens unit II is made of a medium having apositive refractive power and spherical aberration is corrected,differently from the case of the Embodiment 2, by lowering the imageside surface of the second lens unit II.

The Embodiment 4 has the composition illustrated in FIG. 4, wherein thevari-focal lens system according to the present invention comprises, inthe order from the object side, a first lens unit I having a positiverefractive power, a second lens unit I having a positive refractivepower and a third lens unit II having a negative refractive power, andthe lens element arranged at the image side location in the second lensunit II is designed as the GRIN lens element satisfying the conditions(1) and (2).

Like the Embodiment 3, the Embodiment 4 is designed as a vari-focal lenssystem consisting as a whole of six lens elements and so compact as tohave a telephoto ratio of 1.38 at the wide position thereof.

In the Embodiment 4, the lens element arranged at the object sidelocation in the second lens unit II is designed as a GRIN lens elementwhich is made of a medium having a negative refractive power differentlyfrom the case of the Embodiment 3. This medium having the negativerefractive power allows to lower curvature on the object side surface ofthe second lens unit II so as to reduce the angle of incidence of theoffaxial ray, thereby correcting coma favorably. Though the positiverefractive power of the second lens unit II becomes insufficient whensaid GRIN lens element is made of the medium having the negativerefractive power, the insufficiency of the positive refractive power iscompensated by thickening the GRIN lens element arranged at the imageside location in the second lens unit II.

The Embodiment 5 has the composition shown in FIG. 5, wherein thevari-focal lens system according to the present invention comprises, inthe order from the object side, a first lens unit I having a positiverefractive power, a second lens unit II having a positive refractivepower and a third lens unit III having a negative refractive power, andthe lens element arranged at the image side location in the second lensunit II is designed as the GRIN lens element satisfying the conditions(1) and (2).

Like the Embodiment 4, the Embodiment 5 is designed as a vari-focal lenssystem consisting as a whole of six lens elements and so compact as tohave a telephoto ratio of 1.37 at the wide position thereof. Further,the third lens unit II used in the Embodiment 5 consists of two lenselements and comprises an aspherical sur face for obtaining a telephotoratio of 0.96 at the tele position, incontrast to a telephoto ratio of1.01 at the tele position of the Embodiment 4.

The aspherical surface used in the Embodiment 5 has a shape whichweakens a negative refractive power as the portions of said asphericalsurface are farther from the optical axis, and serves for favorablycorrecting the positive distortion produced especially at the wideposition and coma over the entire vari-focal range.

When the direction of the optical axis is taken as the x axis and thedirection perpendicular to the optical axis is taken as the y axis, theshape of the aspherical surface is expressed by the formula shown below:##EQU1## wherein the reference symbol r represents the radius ofcurvature on the reference sphere of said aspherical surface, thereference symbol p designates the conical constant and the referencesymbol A₂.sbsb.i denotes the aspherical surface coefficient.

The GRIN lens element used as the negative lens element arranged at theobject side location in the second lens unit II of the Embodiment 5 hasthe same functional effect as that of the GRIN lens element used in theEmbodiment 4.

The Embodiment 6 has the composition illustrated in FIG. 6, wherein thelens unit having a positive refractive power, which corresponds to thesecond lens unit of the other embodiments, is divided, in the order fromthe object side, into a lens unit having a negative refractive power andanother lens unit having a positive refractive power, and the vari-focallens system according to the present invention consists of four lensunits. Speaking more concretely, the vari-focal lens system preferred asthe Embodiment 6 comprises, in the order from the object side, a firstlens unit I having a positive refractive power, a second lens unit IIhaving a negative refractive power, a third lens unit II having apositive refractive power and a fourth lens unit IV having a negativerefractive power.

In the Embodiment 6, the third lens unit III having the positiverefractive power, out of the lens units which correspond to the secondlens unit II in each of the Embodiments 1 through 5, is composed of asingle GRIN lens element satisfying the conditions (1) and (2).

Since the freedom for correction of aberrations can be enhanced bycomposing the vari-focal lens system according to the present inventionof the four lens units , tile Embodiment 6 is designed as a vari-focallens system which consists of six lens elements having no asphericalsurface, and is so compact as to have a telephoto ratio of 1.30 at thewide position and a telephoto ratio of 0.95 at the tele position.

Further, in the Embodiment 6, the second lens unit II also uses a GRINlens element which has the same functional effect as that of the GRINlens element arranged at the object side location in the second lensunit of the Embodiment 4.

As is understood from the foregoing description, the vari-focal lenssystem according to the present invention consists of three positive,positive and negative lens units or four positive, negative, positiveand negative lens units, and uses at least one GRIN lens element in thesecond positive lens unit of the three lens units or in the thirdpositive lens unit of the four lens units, whereby the vari-focal lenssystem has a vari-focal ratio of 3 or higher, aberrations correctedfavorably over the entire vari-focal range from the wide position to thetele position and a short total length, and consists of a small numberof lens elements.

I claim:
 1. A vari-focal lens system comprising, in order from an objectside:a first lens unit having a positive refractive power; a second lensunit having a positive refractive power disposed so as to be separatedfrom said first lens unit by only an airspace; and a third lens unithaving negative refractive power and disposed at an image side of saidsecond lens unit; wherein said first lens unit consists of two lenselements disposed with an airspace reserved therebetween; and whereinfocal length of said vari-focal lens system is varied by moving each ofsaid lens units along an optical axis of said vari-focal lens system. 2.A vari-focal lens system according to claim 1, wherein said airspacebetween said two lens elements has a minimum thickness when measured onan optical axis of said vari-focal lens system; andwherein focal lengthof said vari-focal lens system is varied by moving each of said lensunits along said optical axis of said vari-focal lens system.
 3. Avari-focal lens system consisting essentially of, in order from anobject side:a first lens unit having a positive refractive power; asecond lens unit having a positive refractive power; and a third lensunit having negative refractive power; wherein said first lens unitconsists of two lens elements disposed with an airspace therebetween;and wherein focal length of said vari-focal lens system is varied bymoving each of said lens units along an optical axis of said vari-focallens system.
 4. A vari-focal lens system according to claim 3, whereinsaid airspace between said two lens elements has a minimum thicknesswhen measured on an optical axis of said vari-focal lens system;andwherein focal length of said vari-focal lens system is varied bymoving each of said lens units along said optical axis of saidvari-focal lens system.
 5. A vari-focal lens system comprising:a firstlens unit having a positive refractive power; a second lens unit havinga positive refractive power; and a third lens unit having a negativerefractive power; wherein said first lens unit consists of two lenselements disposed with an airspace reserved therebetween; and saidsecond lens unit comprises a negative lens element having a concavesurface at an object side thereof and said negative lens element isdisposed at the most object side in said second lens unit; wherein focallength of said vari-focal lens system is varied by moving each of saidlens units along an optical axis of said vari-focal lens system.
 6. Avari-focal lens system according to claim 5, wherein said airspacebetween said two lens elements had a minimum thickness when measured onan optical axis of said vari-focal lens system; andwherein focal lengthof said vari-focal lens system is varied by moving each of said lensunits along said optical axis of said vari-focal lens system.
 7. Avari-focal lens system comprising, in order from the object side;a firstlens unit having a positive refractive power; a second lens unit havinga positive refractive power disposed so as to be separated from saidfirst lens unit by only an airspace; and a third lens unit having anegative refractive power disposed at an image side of said second lensunit; wherein said first lens unit consists of, in order from the objectside, a negative lens element, an airspace, and a positive lens element.8. A vari-focal lens system according to claim 7, wherein focal lengthof said vari-focal lens system is varied by moving each of said lensunits along an optical axis of said vari-focal lens system.
 9. Avari-focal lens system consisting essentially of, in order from theobject side:a first lens unit having a positive refractive power; asecond lens unit having a positive refractive power; and a third lensunit having a negative refractive power; wherein said first lens unitconsists of, in order from the object side, a negative lens element, anairspace, and a positive lens element.
 10. A vari-focal lens systemaccording to claim 9, wherein focal length of said vari-focal lenssystem is varied by moving each of said lens units along an optical axisof said vari-focal lens system.
 11. A vari-focal lens systemcomprising:a first lens unit having a positive refractive power; asecond lens unit having a positive refractive power; and a third lensunit having a negative refractive power; wherein said first lens unitconsists of a negative front subunit and a positive rear subunit; andwherein said second lens unit comprises a negative lens element having aconcave surface on an object side thereof and that is disposed at themost object side in the second lens unit.
 12. A vari-focal lens systemaccording to claim 11, wherein focal length of said vari-focal lenssystem is varied by moving each of said lens units along an optical axisof said vari-focal lens system.
 13. A vari-focal lens system accordingto claim 12, wherein said first lens unit consists of, in order form theobject side, a negative subunit and a positive subunit.
 14. A vari-focallens system according to claim 13, wherein said second lens unitsconsists of two lens elements.
 15. A vari-focal lens system according toclaim 13, wherein said second lens unit consists of three lens elements.16. A vari-focal lens system according to claim 13, wherein said thirdlens unit consists of two lens elements.
 17. A vari-focal lens systemaccording to claim 14, wherein said third lens unit consists of two lenselements.
 18. A vari-focal lens system according to claim 13, whereinsaid third lens unit consists of three lens elements.
 19. A vari-focallens system according to claim 14, wherein said third lens unit consistsof three lens elements.
 20. A vari-focal lens system comprising, inorder from an object side;a first lens unit having a positive refractivepower, a second lens unit having a positive refractive power beingdisposed so as to be separated from said first lens unit only by only anairspace; a third lens unit having a negative refractive power beingdisposed at an image side of said second lens unit; wherein said firstlens unit consists of two subunits; wherein said third lens unitcomprises at least one positive lens element; and wherein a focal lengthof said vari-focal lens system is varied by moving each of the lensunits along an optical axis of said vari-focal lens system in anindependent manner.
 21. A vari-focal lens system according to claim 20,wherein said second lens units consists of two lens elements.
 22. Avari-focal lens system according to claim 20, wherein said second lensunit consists of three lens elements.
 23. A vari-focal lens systemaccording to claim 21, wherein said third lens unit consists of two lenselements.
 24. A vari-focal lens system according to claim 21, whereinsaid third lens unit consists of three lens elements.